The present invention is generally directed to a sensor for detecting suspended particles and, more particularly, to a compact particle sensor.
Obscuration sensors have been utilized as smoke detectors in closed structures such as, houses, factories, offices, shops, ships and aircraft to provide an early indication of fire. Historically, obscuration sensors have included an obscuration emitter and a light receiver spaced at a substantial distance, such as one meter or across a room, to achieve a desired sensitivity. In general, the longer the light beam path, the more likely a smoke particle will interrupt the beam and, hence, the more sensitive the obscuration sensor. Thus, there has been a tradeoff between sensitivity and compactness.
Obscuration sensors have normally been utilized to detect black smoke with particles in the range of 0.05 to 0.5 microns, which are generally produced by rapidly accelerating fires. Traditionally, obscuration or direct sensors have aligned an obscuration emitter and a light receiver such that light generated by the emitter shines directly on the receiver. When a fire exists, smoke particles interrupt a portion of the beam thereby decreasing the amount of light received by the light receiver.
A scatter sensor, commonly known as an indirect or reflected detector, is another type of sensor that has been utilized to detect smoke. A typical scatter sensor has a scatter emitter and a light receiver positioned on non-colinear axes such that light from the emitter does not shine directly onto the receiver. In smoke detectors that have included a scatter sensor, the smoke detector has included a test chamber that admits a test atmosphere, while at the same time blocking ambient light. A light receiver within the test chamber receives light provided by an emitter located within the chamber. The light level received provides an indication of the amount of smoke in the test atmosphere. Smoke particles in a test chamber reflect or scatter light from the emitter to the receiver. Most scatter sensors generally work well for gray smoke but have a decreased sensitivity to black smoke.
Obscuration sensors have been proposed that utilize a mirror within a test chamber to reflect a light beam provided by an obscuration emitter to increase the path length traveled by the light beam to improve the overall sensitivity of the obscuration sensor. In this type of obscuration sensor, the emitter and the receiver have not been located on the same axis. That is, the emitter and the receiver have been located on non-colinear axes such that light from the emitter did not shine directly onto the receiver. However, proposed obscuration sensors that have implemented a mirror have incorporated the mirror and the components in the same plane, which would yield an apparatus with relatively large dimensions in order to achieve a desirable sensitivity. Further, such sensors have implemented fixed alarm thresholds and, as such, have generally been incapable of adapting to changing environmental conditions and responding appropriately to different particle reflectivities.
What is needed is a sensitive, low cost, compact particle sensor that is equally sensitive to both low and high reflectivity particles that can be implemented within a relatively small volume.